Our research program is concerned with elucidating the neural structures associated with memory acquisition, storage, and retrieval. We have continued to concentrate on three domains of memory. The first domain focuses on semantic memory, the part of long-term memory composed of general information, such as facts, ideas, and the meaning of objects and words. We are particularly interested in characterizing the neural substrate mediating object and word meaning and its role in object perception. We are also interested in understanding how abstract forms of knowledge, such as knowledge of social interactions, are represented. The second domain focuses on repetition priming, a form of implicit learning not dependent on conscious retrieval of prior events. These studies concentrate on characterizing changes in neural activity associated with perceptual skill learning to provide a model system for studying cortical plasticity. The third domain, episodic memory, is an explicit form of memory that involves conscious recollection of a specific past experience. These studies concentrate on identifying the brain structures unique to this form information retrieval. The results of our previous functional brain imaging studies on semantic memory using positron emission tomography (PET), suggested that object knowledge is stored in distributed networks of discrete cortical regions. In addition, the location of these regions parallels the organization of sensory and motor systems in the brain. These findings have been extended by a recent series of investigations of object processing using functional magnetic resonance imaging (fMRI). These studies have revealed fine-grained differences between a number of object categories (animals, tools, faces, houses) along both the ventral (especially the fusiform gyrus), and lateral (middle temporal gyrus, superior temporal sulcus) regions of the temporal lobes. The results of these studies have been consistent with the idea that information about object form, and form-related features such as color, is stored in ventral regions of the temporal cortex, anterior to sites in occipital cortex that mediate perception of these features, whereas information about object motion is stored in lateral temporal regions, anterior to areas in occipital cortex that mediate perception of motion. In addition, activity in the left premotor cortex and left intraparietal sulcus - both of which are active when objects are manually manipulated - is enhanced during recognition and naming of pictures of tools, relative to other objects that are not typically manipulated by hand. These results provide additional evidence that information about the defining features of objects is stored within the sensory and motor systems that were active when we first learned about those objects. Our studies of implicit (perceptual priming) and explicit (recognition) memory processes have continued to explore how neural activity is modulated by experience. Behavioral studies have shown that objects are named faster if they have been previously seen. In addition, this facilitation has been found when as much as a year intervenes between the first and second occurrence of the object, suggesting a permanent change. In a series of studies we showed that a single, brief (200 msec), presentation of an object leads to fast developing (within 30 sec) and long-lasting (up to 3 days) decreases in neural activity in posterior cortex, coupled with slowly developing (over a 3 day period) decreases in left inferior prefrontal cortex and increases in insular cortex. These findings suggest that time-dependent changes in posterior and anterior cortices may underlie perceptual and lexical/semantic forms of learning that occur whenever objects are identified and named. Explicit recognition of an item is also associated with reduced activity in posterior cortices, suggesting that this repetition-related reduction in neural activity is an automatic response to the repeated presentation of an object. In contrast to priming, explicit recognition memory is associated with increased activity in several regions of prefrontal cortex that are not active during performance of implicit memory tasks.